1. Field of the invention:
This invention relates to a semiconductor laser device and more particularly to a semiconductor laser device with high reliability that can be produced by vapor phase epitaxy such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MO-CVD), gas source molecular beam epitaxy (GS-MBE), etc.
2. Description of the prior art:
In recent years, an epitaxial growth technique for the formation of thin films such as MBE, MD-CVD, GS-MBE, etc., has been developed which enables the formation of epitaxially grown thin layers having a thickness of as thin as approximately 10 .ANG. or less that is the order of a monolayer. In the field of semiconductor laser devices, the development of such an epitaxial growth technique, although these significantly thin films have not yet been produced by the conventional growth technique that is carried out under conditions where thermal equilibrium is approximately established, such as liquid phase epitaxy (LPE), vapor phase epitaxy in which hydrides or chlorides are used as starting materials, etc., allowed a device structure with significantly thin films to be applied to laser devices, resulting in semiconductor laser devices with improved device characteristics. For example, when the conventional LPE is used, it is difficult to form a double heterostructure uniformly over the entire surface of a wafer while maintaining a steep gradient of the composition of semiconductor materials at the interface of the heterojunction. This construction of semiconductor laser devices can readily be achieved by the use of MBE or MO-CVD. Also, the higher limit of output power attained by a semiconductor laser device with a double heterostructure is dependent mainly on the intensity of laser light in an active layer thereof. As mentioned above, the active layer can readily be produced so as to have a thickness of 500 .ANG. or less by the use of MBE or MO-CVD. Therefore, the laser light can permeate into a cladding layer thereof, and thus the intensity of laser light in the active layer decreases, so that a higher limit of output power can be improved.
The vapor phase epitaxy such as MBE, MO-CVD, etc., have the excellent advantages mentioned above and can attain the uniform formation of epitaxially grown layers over a large wafer with a diameter of 2 inches or more. However, these techniques have not been widely applied to mass production of semiconductor laser devices because the reproducibility of crystals with sufficient quality to provide semiconductor laser devices with high reliability is invariably poor.
Conventional semiconductor laser devices have a device structure that is epitaxially formed on the (100) plane of a semiconductor substrate. When the above-mentioned growth techniques that are carried out under conditions where thermal equilibrium is not achieved are used for the formation of such a device structure, the control of crystallinity cannot readily be attained, so that semiconductor laser devices with high reliability cannot be produced with good reproducibility and in good yield. That is, for example, when these techniques are used for the production of a group III-V compound semiconductor laser device, the ratio of the group V to the group III in the layer to be grown should be controlled so as to be constant, in order to reduce vacancies at the positions of the group III atoms and the group V atoms or point defects such as interstitials, etc.
When LPE is used, a semiconductor layer that is being grown is covered with the melt containing the group III atoms or the group V atoms, so that the semiconductor layer is not contaminated by large amounts of oxygen, water, hydrocarbon, etc., that are contained in a surrounding atmosphere. In contrast, when vapor phase epitaxy such as MBE, MO-CVD, etc., is used, it is difficult to maintain the surrounding atmosphere clean, so that these impurities are directly incorporated into the semiconductor layer from a vapor phase. These difficulties are responsible for a decrease in the reproducibility of semiconductor laser devices with high reliability.
Moreover, when LPE is used, the semiconductor layer is grown on a substrate disposed horizontally by the use of a sliding-boat method, so that there is no large stress applied to the substrate during the growth. However, when the substrate is fixed by the use of metallic indium (In) in carrying out MBE, the substrate carries stress by subjecting to the alloying reaction with the molten In and/or by solidifying the molten In with a decrease in temperature after growth. Also, when the substrate is mechanically fixed without using In in carrying out MBE, or when the substrate is inclined, in order to improve the uniformity of growth or in order to perform the growth on a plurality of substrates, the substrate carries stress on the fixed part thereof, which causes the generation of dislocations in the epitaxial layers grown on the substrate during the growth. The dislocations generated by the stress result in a poor yield of semiconductor laser devices with high reliability.
Recently, the inventors of this invention found the fact that the quantum effect of a semiconductor device in the &lt;111&gt; direction is greater than that of a conventional semiconductor device in the &lt;100&gt; direction. Based on this fact, the inventors succeeded in the production of a semiconductor device having excellent device characteristics by the use of the quantum effect in the &lt;111&gt; direction in which a device structure of the said device is formed on the (111) plane of a substrate (U.S. patent application Ser. No. 159,797).